Um, Yes,
I guess you are using a tube amp. If a radio station transmitter is close by you can pick up the signal because the signal is radiating right into a tube. Tubes, as well as other parts of the amp are shielded with metal housing surrounding the tubes, among other things. See if one is missing. If so you will have to see if you can find one. Places that sell tubes may also be able to supply you with tube shields.


Ray E. Strode